In mass culture of pluripotent stem cells such as human iPS (induced pluripotent stem) cells, a series of amplification culture (subculture) processes are repeated so that many undifferentiated cells are prepared. It is known that in this series of culture processes, cells deviated from the undifferentiated state, that is, “deviated cells”, spontaneously emerge.
It is known that the deviated cells have a division potential that is nearly equivalent to that of undifferentiated cells, and induce the conversion from undifferentiated cells to deviated cells. In other words, when deviated cells emerge, the proliferation rate thereof exceeds that of undifferentiated cells, and the proliferation of undifferentiated cells is suppressed.
The emergence of deviated cells is frequently observed in culturing operations performed by unskilled culture operators. Further, an excessively large colony size and fusion of colonies are known to be factors of the emergence. Therefore, subculture at low confluence, and maintenance of uniformity at seeding, make it possible to decrease the frequency of appearance of deviated cells to some extent. Further, by using a medium developed in recent years, the frequency of appearance of deviated cells is suppressed to some extent. Deviated cells, however, still spontaneously emerge, and in the case where the cells emerge, it is still essential to remove colonies that contain deviated cells.
In order to maintain the undifferentiated state, colonies containing deviated cells are carefully removed by pipetting operations under a microscope apparatus upon subculture. A device performing such an operation of removing colonies, for example, an observing device combined with pipetting by robot handling, has been developed as well.
Further, Patent Document 1 discloses culture of pluripotent stem cells in the presence of activin for proliferating pluripotent stem cells such as iPS cells while maintaining undifferentiated states thereof.